Wish I could help, but you are doing the same as I would. Take a sample and go to the Vet. Did the breeder say anything as that would be another call I would make?
Have you had a lot of humidity? That can cause stress on animals as well and could cause bellies to go sour? This was the time of year Jake would eat grass to help balance his belly flora. I know you are watching him like a hawk, so don't think it was anything that he ate that was foreign to him.
Fingers crossed you find a solution for Bruce. Poor boy.
KAZ